This invention relates to a method of producing a photomask for use in production of a semiconductor integrated circuit, a liquid crystal display device, or the like, and to a photomask blank for use therein.
Following high integration of semiconductor integrated circuits, liquid crystal display devices, etc., and so forth, there has been an increasing demand for high pattern accuracy with respect to photomasks that are used in fine processing during the production process thereof.
In the photomasks currently used, a pattern formed by an opaque film is provided on a transparent substrate, and a chromium-based material is generally used as the opaque film in terms of processability of a highly accurate pattern.
However, with respect to the demand for higher accuracy of the patterns of the photomasks following the high integration of the semiconductor integrated circuits and so forth, it has become clear that the current method of forming a pattern from a chromium-based opaque film by using a resist pattern as an etching mask raises a problem of variation in CD (critical dimension) accuracy due to the loading effect in connection with a photomask wherein pattern regions having a global opening ratio difference are compositely present in the mask plane. As the photomask wherein the pattern regions having the global opening ratio difference are compositely present in the mask plane, there is specifically cited a photomask wherein a plurality of kinds of functional devices are disposed in the mask plane. As such a photomask, there is cited, for example, a photomask having patterns with a difference in density and used in production of a system LSI wherein memories, logic circuits, and so forth are compositely mounted, or a D-RAM, a liquid crystal display device, or the like wherein memory cells or pixel regions, and peripheral circuits and so forth formed therearound are compositely mounted. In such a photomask, for example, the opening ratios of opaque patterns (the ratios of portions where no opaque films are formed) differ between a memory region and a logic circuit region. In the current state, in order to produce such a photomask, a desired resist pattern is first formed on a chromium-based opaque film, then, using this resist pattern as a mask, the chromium-based opaque film is patterned by dry etching mainly with radicals by the use of a chlorine-based and oxygen-based mixture gas or the like. For example, when resist patterns having the same dimensions are formed in respective regions having an opening ratio difference in opaque pattern so as to form chromium-based opaque film patterns of the same dimensions, respectively, there arises a problem that the respective opaque patterns formed by dry etching using the resist patterns of the same dimensions as masks exhibit different dimensions due to the difference between the opening ratios in the respective regions by the so-called loading effect to thereby cause variation in CD accuracy.
Here, the loading effect is a phenomenon that etching characteristics (etching rate, selection ratio, etc.) change depending on a magnitude of an etching area of an etching-subject film so that a CD shift in the mask plane changes (see, e.g. VSLI Synthetic Dictionary (Science Forum) pp. 865). More specifically, it is a phenomenon that when the etching area increases, the utilization efficiency of etchants decreases to reduce the etching rate (see, e.g. Submicron Lithography “Synthetic Technological Material Collection” pp. 353).
Generally, in dry etching of a thin film made of a material containing chromium, chlorine and oxygen are used as dry etching gases to etch chromium by producing chromyl chloride. There is a problem that since contribution of radicals is large in this etching reaction, the etching progress directivity is particularly difficult to control.
As a means for solving the problem about the photomask caused by the reduction in CD accuracy due to the foregoing loading effect, there is a method wherein a pattern accuracy in etching is not lowered by improving inequality of opening ratios in a pattern peripheral region and a pattern central portion (see, e.g. Japanese Patent Application Publication (JP-A) No. 2001-183809). Specifically, this method is a method of providing a peripheral opening ratio adjusting pattern in a non-irradiated region where light from a light source is not irradiated in an exposure process using a photomask.
On the other hand, there is also a method of disposing a dummy etching pattern for dry etching rate correction in a pattern exposure region or outside the pattern exposure region when producing a phase shift photomask (see, e.g. Japanese Patent Application Publication (JP-A) No. H8-234410). In this method, as the dummy etching pattern in the pattern exposure region, use is made of a pattern having a size not greater than the resolution limit by transfer.
However, in the conventional methods described in Japanese Patent Application Publication (JP-A) No. 2001-183809 and Japanese Patent Application Publication (JP-A) No. H8-234410, there is a problem that when local inequality exists in the pattern opening ratio (i.e. when pattern regions with different densities compositely exist), it is complicated to provide an adjusting pattern corresponding thereto and it is difficult to cope with high integration of semiconductor integrated circuits. Further, since it is necessary to form the pattern that is not normally required, an increase in pattern data amount used in production of a photomask can not be avoided. This becomes a large problem in producing semiconductor devices with extremely high integration in recent years.
As a literature describing another method of suppressing the loading effect, there is U.S. Pat. No. 6,472,107. This literature describes that the loading effect can be suppressed by etching an opaque film by the use of a hard mask layer made of Ti, TiW, W, Si3N4, SiO2, TiN, spin-on glass, or the like. In this method, it is not necessary to form the pattern, which is not normally required, when producing the photomask like in Japanese Patent Application Publication (JP-A) No. 2001-183809 and Japanese Patent Application Publication (JP-A) No. H8-234410 as described above. Note that the method itself of etching the opaque film by the use of the hard mask layer as described in U.S. Pat. No. 6,472,107 has been proposed for a long time also as a technique of solving a problem of reduction in CD accuracy when use is made of a resist having a small dry etching resistance (see, e.g. Japanese Patent Publication (JP-B) No. S63-39892). In terms of improving the CD accuracy, as another technique of improving the CD accuracy of a chromium film pattern, Japanese Patent Application Publication (JP-A) No. H10-69055, for example, discloses that the CD accuracy is improved by reducing the thickness of a chromium film and the thickness of a resist film that requires about three times the thickness of the chromium film.
Meanwhile, there is a phase shift mask as a photomask apart from the one called a binary mask and having been used for a long time wherein the opaque film pattern is formed on the transparent substrate. The phase shift mask improves contrast of a transferred image by providing a phase shifter portion on the mask and by shifting by 180° the phase of light passing through the phase shifter portion and a portion adjacent thereto to cause mutual interference of the light at a boundary portion therebetween. As the kind of phase shift mask, there are cited, for example, a Levenson type, a halftone type, a chromeless type, and so forth. A phase shifter layer in the Levenson-type phase shift mask is formed by normally etching a glass or is comprised of a film made of a material serving to shift a phase. A phase shifter layer in the halftone-type phase shift mask is comprised of a semitransparent phase shift material layer. These phase shift masks require an opaque ring at a peripheral portion of a pattern region for preventing leakage of exposure light. A chromium-based opaque film is normally used as this opaque ring. In the production of a phase shift mask having such an opaque ring, use is normally made of a blank in which an opaque film is formed on a phase shift material layer. First, the opaque film is etched to form a desired opaque film pattern, then the phase shift material layer is etched using the opaque film pattern as an etching mask for the phase shift layer, and then the opaque film is removed while leaving at least a portion of the opaque film that will serve as the opaque ring, thereby to produce the phase shift mask. By using such a method, the phase shift material layer has achieved a higher CD accuracy as compared to etching using a resist pattern as a mask.
Meanwhile, in recent years, further integration of the semiconductor devices has been advanced, wherein the line width has also been discussed from a line width of up to 130 nm to a line width of 90 nm, 65 nm, and further 45 nm so that higher densification of patterns has also been advanced. Therefore, there is a tendency that patterns of the photomasks are also formed further finer and much stricter values are required also for the CD accuracy thereof. There is also a tendency that since diversification of the patterns has been advanced, differences in density of the patterns also increase.
As described above, the foregoing U.S. Pat. No. 6,472,107 describes that the loading effect is suppressed by using the hard mask, and Japanese Patent Publication (JP-B) No. S63-39892 describes about improving the CD accuracy by using the hard mask. However, in order to suppress the loading effect and to achieve the high CD accuracy under the circumstances where the finer formation of the patterns and the differences in density of the patterns are in progress as described above, it is insufficient to merely use the hard mask and further technological improvement is required.
On the other hand, in terms of the improvement in CD accuracy under the foregoing circumstances, in the method described in Japanese Patent Application Publication (JP-A) No. H10-69055, since the chromium-based opaque film requires a prescribed opaque property (e.g. OD (optical density) of 3.0 or more), there is a limit to reduction in thickness of the opaque film, and resultantly, there is also a limit to reduction in thickness of the resist, and therefore, there is a limit to improvement in CD accuracy.
Further, in the production of the phase shift mask, the pattern shape of the opaque film to serve as the etching mask is directly reflected on the pattern shape of the phase shift material layer, and therefore, a dimension control of the opaque film pattern performs a very important role. In particular, the phase shift mask is a mask that is effective in fine formation of a pattern in a semiconductor device as compared to the binary mask. In recent years, since further finer formation of patterns has been progressed, further stricter dimensional accuracy of phase shift material layers has been required. On the other hand, there is also a problem that, depending on the etching condition of the phase shift material layer, the surface of the chromium-based opaque film is damaged on etching of the phase shift material layer, and particles generated thereby affect the etching of the phase shift material layer and remain as a pattern defect, which thus narrows the width of selection in etching condition.